What is the difference between the two sections of code?

What is the difference between the two sections of code? -- Linux general technology-Linux programming and kernel information. The following is a detailed description. The first code cannot be compiled:
# Include <ctype. h>

Main ()

{

Char str [] = "123c @ # FDsP [e? ";

Int I;

For (I = 0; tr ! = 0; I ++)

If (isalnum (str) Printf ("% c is an alphanumeric character \ n", str);

}

The second code has been compiled:
# Include
Main ()
{
Char str [] = "123c @ # FDsP [e? ";
Int I;
For (I = 0; str ! = 0; I ++)
If (isalpha (str) Printf ("% c is an alphanumeric character \ n", str);
}
Why? The error I can find is "tr" in the loop of the first code"Missing a character" s ", that is," str"", And the second error is in the first line. I haven't understood it for a long time!